It's a good idea, but probably not practical / efficient.
You report lowering your GPU by two degrees, but the topside cooler can yield improvements of 20 degrees (depending on card and cooling solution). That should be a good tip that this idea may not be all that it's worth.
Still it might be worth exploring.
I posted the idea of a CPU backside cooler a long time ago (never actually did it):
http://forums.procooling.com/vbb/showthread.php?t=5438